Such an interferometric measuring device is discussed in German patent document no. 198 19 762. In this measuring device, one part, the so-called modulating interferometer, is spatially separated from the actual measuring probe, and is optically connected to it via a light-conducting fiber system, so that the measuring probe per se may be designed as a relatively simply constructed, easily manipulable unit. A broad-band, short-coherent radiation is supplied to the modulation interferometer, which is split into two beam components, at the input of the modulation interferometer with the aid of a beam splitter, of which the one is shifted in its light phase or light frequency with respect to the other, using a modulation device, such as an acousto-optic modulator.
In the modulating interferometer, one of the two beam components runs through a delay element which generates an optical path difference of the two beam components which is greater than the coherence length of the short-coherent radiation. In the measuring probe, in a measuring arm, with respect to a reference arm, an additional optical path difference is generated in such a way that the path difference effected by the delay element is compensated for, and, consequently, an interference is created of the reference radiation coming from the reference plane of the reference arm and the radiation coming back from the object surface in the measuring branch, which is subsequently analyzed so as to ascertain the desired surface property (shape, roughness, separation distance) via a phase evaluation. In the measuring probe, the measuring arm and the reference arm are situated in one exemplary embodiment in one common light path (common path), a partially transmitting element being provided for forming the measuring arm and the reference arm.
A similar interferometric measuring device having such a modulating interferometer and a measuring probe connected to it via a light-conducting fiber system is also discussed in German patent document no. 198 08 273, in a beam splitting and radiation detecting unit, using receiving equipment, a splitting of the radiation brought to interference into radiation components of different wavelength taking place, so as to form therefrom a synthetic wavelength and to increase the measuring range (range of unambiguity).
In the interferometric measuring devices named above, which are based on heterodyne interferometry, but which utilize the properties of a broad-band, short-coherent radiation, the modulating interferometer designed as a Mach-Zehnder interferometer has a system of classical optical components, such as collimation optics lying upstream of the input end of the beam splitter, the beam splitter and reflecting mirror at the input end and the output end. In this context, the beam components experience several reflections at the beam splitter surfaces and at the mirrors, before they are coupled in to the optical light-conducting fiber system. The optical components have to be positioned with great accuracy, since the effect of every angle error is doubled by the reflection. In this context, it is difficult to ensure the durability of a calibration. In connection with fitting in a glass plate to compensate for optical asymmetries, too, additional difficulties come about during the calibration. A costly construction is connected with these difficulties, an exact adjustment to the properties of the measuring probe being also required.